Wavelength division multiplexing (WDM) has been explored as an approach for increasing the capacity of fiber optic networks to support the rapid growth in data and voice traffic applications. A WDM system employs plural optical signal channels, each channel being assigned a particular channel wavelength. In a WDM system, signal channels are generated, multiplexed, and transmitted over a single waveguide, and demultiplexed to individually route each channel wavelength to a designated receiver. Through the use of optical amplifiers, such as doped fiber amplifiers, plural optical channels are directly amplified simultaneously, facilitating the use of WDM systems in long-distance optical systems.
Recently, switching elements that provide a degree of reconfigurability have become available. These reconfigurable optical elements can dynamically change the path along which a given wavelength is routed to effectively reconstruct the topology of the network as necessary to accommodate a change in demand or to restore services around a network failure. Examples of reconfigurable optical elements include optical Add/Drop Multiplexers (OADM) and Optical Cross-Connects (OXC). OADMs are used to separate or drop one or more wavelength components from a WDM signal, which is then directed onto a different path. In some cases the dropped wavelengths are directed onto a common fiber path and in other cases each dropped wavelength is directed onto its own fiber path. OXCs are more flexible devices than OADMs, which can redistribute in virtually any arrangement the components of multiple WDM input signals onto any number of output paths. FIG. 1 shows a conventional cross-connect 100 that has two input ports 1011 and 1012 and output ports 1031 and 1032, which can each communicate a WDM signal having N channels or wavelengths λ1-λN. Each WDM input and output port is coupled to a demultiplexer and multiplexer, respectively. Specifically, cross-connect 100 includes demultiplexers 1051 and 1052, and multiplexers 1071 and 1072. Cross-connect 100 also includes M×M switching fabric 109, where M is equal to N times the number of WDM input/output ports (m). In the example shown in FIG. 1, M is equal to 2N. Switching fabric 109 is traditionally an electronic switching core such as a digital cross-connect, however for current high capacity optical systems this is being replaced with an optical switching system.
Unfortunately, because current OXC's optical switches have a relatively high insertion loss, they require optical-to-electrical interfaces and regenerators into and out of the cross-connect. While these regenerators overcome the problem of insertion loss and effectively allow wavelength conversion of the signal as it traverses the switching fabric, they substantially add to the cost of an already expensive switching fabric because a regenerator is required for each and every wavelength that is used in the network.
Another limitation of the aforementioned conventional OXC is that it is difficult to increase the number of input and output ports when such additional capacity is needed sometime after the OXC is initially installed and operational. In order to provide such modularity, the switching fabric 109 as initially installed must include its maximum anticipated capacity, because otherwise the loss and number of connections increase too rapidly. In other words, it is impractical to provide an MxM switching fabric that is itself modular. This limitation may be mitigated to a small degree by packaging demultiplexers and monitoring detectors outside the MxM switching fabric in modules that can be installed incrementally, but since the switching fabric is the most expensive component in the OXC, the advantages of providing a conventional OXC that is modular are limited.
Accordingly, it would be desirable to provide a low-loss optical cross-connect in which modular functionality can be provided in a relatively easy and inexpensive manner.